Recently, hybrid electric rotary excavators have been being developed, in which a rotary body is driven by an electric motor while a work machine and a carrier are driven by a hydraulic actuator (see, for instance, Patent Document 1).
Since the rotation of the rotary body is driven by the electric motor in such electric rotary excavators, even when the rotary body is rotated while a boom and an arm that are driven hydraulically are lifted up, the rotation of the rotary body is not affected by the lifting of the boom and the arm. Accordingly, an energy loss at control valves or the like can be reduced as compared to typical hydraulic excavators in which the rotary body is also driven hydraulically, thereby enhancing energy efficiency.
Meanwhile, in typical hydraulic excavators, the rotary body is also driven by hydraulic pressure from a hydraulic pump like the work machine, the hydraulic pump being driven by an engine. Due to the arrangement, by changing an amount of fuel supply to the engine to adjust engine speed, discharge rate of hydraulic oil from the hydraulic pump changes accordingly, thereby changing rotation speed of the rotary body. Specifically, by turning a fuel dial to reduce the amount of fuel supply, the engine speed decreases, thereby decreasing the rotation speed of the rotary body accordingly. In contrast, by turning the fuel dial to increase the amount of fuel supply, the engine speed increases, thereby increasing the rotation speed of the rotary body accordingly.
In such hydraulic excavators, change of the amount of fuel supply to deliberately adjust the engine speed is performed also by operating a mode selection switch for selecting a work mode in addition to operating the fuel dial. The work mode includes an active mode, an economy mode, a breaker mode, a lift mode and the like in descending order of the engine speed, the work mode selected properly depending on a work to be performed.
[Patent Document 1] JP-A-2001-11897